The term "carrying fabric" as used herein is intended to include wires, screens, belts, and other web supporting means, typically made from plastics such as polyesters.
Carrying fabrics are often used in the dryer sections of paper machines or other web processes where the web is wet and lacks strength and must be supported. However, a carrying fabric is expensive not only in capital cost but also in the downtime which results when the machine must be serviced and the fabric replaced.
Accordingly, any improvement which would extend the useful life of a carrying fabric is of principal concern in the paper and textile industries and any industry where webs and films are employed.
In a through-air dryer of the rotary type, heat-generating means is used to heat and dry the web as it passes over a roll. In this system, air is drawn through the web (e.g., paper) as the result of a pressure differential across the web surface and moisture is removed by convective heat and mass transfer between the web and the air.
The roll in this system includes a cylindrical wall which is air permeable and which serves as a support for the carrying fabric. The carrying fabric, in turn, carries and supports the web. Two nip rolls hold the carrying fabric and determine the extent to which the web wraps around the roll.
The heated air for drying the web may flow from outside the cylinder to the inside as, for example, by passing the web under a heating hood or, alternatively, it may flow from in side the cylinder to the outside. In either case, the effect on the carrying fabric is the same because the heated air drawn through the roll exposes the edge of the carrying fabric to the degrading effects of unremittingly high operating temperatures.
The drying time for the web depends upon the desired degree of drying, the speed of the line, the amount of heat applied and the volume of air which is drawn through the fabric. Since it is more energy efficient and cost effective to heat air than move it, it is preferable to increase the operating temperature of the dryer and minimize air volume. Moreover, as the line speed increases, so does the need to remove water. Therefore, it is desirable to operate dyers at higher temperatures as the line speed increases.
Operating a dryer at higher temperatures, however, reduces the life of the carrying fabric because it thermally degrades the fabric's edge. Fabric degradation is a function of fabric temperature and the period of time over which the fabric is exposed to high temperatures. Accordingly, any means which provides for a decrease in fabric temperature and limits the exposure of the fabric to the degrading effects of high temperature will extend fabric life.
In a typical system, for example, hot air is supplied at temperatures in the range of from about 350-520.degree. F.; however, when this heated air passes through the wet web, it picks up moisture and the temperature is greatly reduced. As a result, the air which passes through the web-covered fabric is in the range of about 190-250.degree. F., whereas, the temperature of the air which passes through those fabric are as not covered by the web is usually within the 350-520.degree. F. range. Accordingly, those are as of the carrying fabric which are not covered by the web receive the impact of high heat and suffer its degrading effect. On the other hand, those are as of the carrying fabric which are web-covered are protected because the moisture in the web exerts a cooling effect which is transmitted to the fabric's interior.
Accordingly, unless adequate cooling is provided, the exposed edge of the carrying fabric will be thermally degraded and the useful life of the entire fabric will be severely limited.
Moreover, excessively high temperatures may even cause the fabric to melt and/or catastrophically fail and this can result in significant downtime and expense.
Accordingly, there is a need to extend the life of a carrying fabric by moderating the temperatures to which its exposed edges are subjected during the drying operation.
Various devices and methods for reducing the temperature of a fabric's edge are known in the art but they are unsatisfactory. One such method directs a "curtain" of cool air along the edge of the fabric. This cool air, typically ambient air, is blown down from a separately contained supply section housed within a hood and it cools the edge and protects it from the otherwise hot air that blows over the web.
Another method provides for the application of suction means to the edge of the roll so that ambient air can be drawn in from the side between the hood and roll. As the ambient air passes through the fabric, it cools the edge.
Still another method prevents suction from being drawn over the edge of the roll by placing a sleeve under the exposed edge of the fabric. The reasoning is that, since the hot air which emanates from the hood is prevented from passing through the roll, it cannot pass through the carrying fabric. The convective heat transfer between the air and fabric is therefore less than it would be if the air were allowed to pass through the fabric. Consequently, the rate of rise of fabric temperature is less when this method is employed.
Another method for preserving the carrying fabric is simply to reduce the operating temperature of the dryer.
Although these methods are of some benefit in reducing degradation of the fabric's edge, each has its shortcomings.
For example, the use of ambient air to shield or cool the exposed edge of a fabric has limited value because air has a limited capacity to absorb heat. The alternative is to reduce the temperature at which the dryer may operate; however, the result is a reduction in line speed.
Also, the use of a sleeve to prevent hot air from passing through the carrying fabric will moderate only slightly the effects attributed to heat and the surface of the fabric will still remain exposed to the degrading effects of high temperatures.
Moreover, it has been found that the level to which temperatures must be reduced to dry paper and minimize damage to the carrying fabric, is of such magnitude that either the air volume must be increased or the line must be slowed. However, an increase in drying air volume can only be achieved by the use of motors and electricity and this is less cost effective than using fuels such as natural gas or oil to operate at higher temperatures. Since, however, the object in web processing is to maximize paper production, any slowing of the line speed would be counterproductive.
Accordingly, a need exists for protecting the exposed edge of a carrying fabric by minimizing the effects of thermal degradation while at the same time, avoiding any increase in air volume or decrease in line speed.